1. Field of Invention
This invention is directed to apparatus and methods that convert input image data into device-biased image data for use with one or more image output devices.
2. Description of Related Art
When a user creates a document or image having, for example, a plurality of colors, the colors each have values associated with them in order to categorize the color in such a way that the color may be reproduced. For example, with a cathode ray tube (CRT) device, computer monitor, and other such similar display devices, the colors of individual pixels are identified in terms of the red (R), green (G) and blue (B) components of the color to be displayed for the pixel. For devices that provide hard copy images, such as for example, printers, photocopiers, and the like, the colors that are output are identified by their cyan (C), magenta (M), yellow (Y) and black (K) components.
To make it convenient to handle many different input and output devices, it is becoming more common to describe the colors in a xe2x80x9cdevice-neutralxe2x80x9d (also called device-independent) color space. This color space essentially describes how the color is seen by the eye and typically uses color spaces originally standardized by the CIE in their 1931 or later standards. In recent standards by the International Color Consortium, these spaces are also referred to as Profile Connection Spaces (PCS), where the profiles describe how given device color descriptions are transformed into (or from) the PCS. The device-neutral space referred to in the following as the exemplary space is L*a*b*, defined by CIE, where L* is the lightness and a* and b* are color differences from gray (roughly described as red-green and blue-yellow). In general, conversions to/from these spaces require multi-dimensional conversions, usually done in computers by three or four dimensional lookup tables (LUTs).
When an image is to be output by an image output device, the image typically must be converted from device-neutral image data, such as L*a*b*, to device-specific image data, such as cyan (C), magenta (M), yellow (Y) and black (K), for the particular output device. Alternatively, if the original data is specific to the input device, then calibration data is needed and the equivalent of the transformation from input device to device-neutral to output device-specific output is done as one very complicated transformation. In either case, this resulting device-specific image data is stored as the image data for the image. Storing device-specific image data as the image data for the image decreases the processing time necessary for processing the image data when the image is to be output because the image data is already in a form useable by the image output device. However, the conversion from device-neutral image data to device-specific image data is irreversible.
One reason that the conversion is irreversible is that not all devices are capable of displaying or printing the same color values. The image output device outputs the image using device-specific image data which has been obtained from device-neutral image data. During the conversion from device-neutral image data to device-specific image data, because not all color values in the device-neutral image data color space can be accurately represented in the device-specific image data color space, some color values are lost during the conversion. This is due to limiting the ranges of color values to convert them into color values that are within the gamut of the image output device. Thus, since the original color values have been lost during the conversion, a reverse conversion is not capable of recapturing these color values.
One way to overcome this problem is to store the image data as device-neutral image data and to perform the conversion into device-specific image data whenever the image is to be output by an image output device. However, because the conversion is complex, the computing time to effect the conversion is great. Thus, the processing time necessary to process the image data for output is greatly increased. This method is acceptable, even preferable, when the data are output to different devices each time output is desired, but it is much less desirable when the data may be output numerous times on the same, or very similar, devices. It is the latter case which causes people to resort to the time-saving method of saving device-specific data forms, with the result that information has been lost and the best reproduction can no longer be achieved on any other output device.
Another way to overcome the above problem in the prior art is to store the image as both device-neutral and device-specific image data. However, this effectively doubles the amount of memory needed to store the image. Furthermore, if more than one type of image output device is being utilized, multiple versions of the device-specific image data may be necessary and would thus, increase the memory requirements even more. Therefore, maintaining the image as both device-neutral and device-specific image data is not a practical solution.
This invention provides a solution to the problem of desiring a method of storing image data such that a given (or similar) output device can quickly and easily output the data, such as for numerous reprintings, yet maintaining the capability to perform a complete and accurate reverse transformation to the original device-neutral data, to enable a full quality output to a completely different output device. The method provides solutions which require much less storage space than the previously known method of keeping both the device-neutral and device-specific forms.
This invention provides apparatus and methods for providing xe2x80x9cdevice-biasedxe2x80x9d image data for use with one (or similar) image output devices. The device-biased image data has the properties that (1) it may be easily converted into device-specific image data, without any multi-dimensional transformations, and (2) it contains unique descriptions of all original colors, such that an accurate reverse transformation to the original device-independent form is possible.
The apparatus according to this invention may be incorporated into an image processing system that includes an image data source, a device-biased image data converter and an image output device. The device-biased image data converter performs device-biased color space adjustment operations on the image data input by the image data source. The device-biased image data converter converts the input image data into device-biased image data according to one or more conversion methods and stores the device-biased image data in memory in place of the input image data. The device-biased image data may then be converted into device-specific image data whenever the image is to be output by an image output device.
Because the image data is stored as device-biased image data, large multidimensional look-up tables are not required to convert from the device-biased image data to device-specific image data. Rather, for example, one dimensional look-up tables may be used to convert device-biased image data to device-specific image data. This is because the image data has already been mostly converted into device-specific image data by converting the image data into device-biased image data. Thus, with the use of the device-biased image data converter, image data may be stored in a device-biased color space which is able to be quickly converted to device-specific image data without requiring large amounts of processing time or memory space and is also capable of being fully restored to the original, device-neutral, form.